Preheat oven to 350 degrees.
Grease a 13" x 9" baking dish.
Boil the pasta in water until two minutes before recommended cooking time, then drain and set aside.
Heat the cream of mushroom soup and milk in a saucepan on medium heat for 2 minutes.
Stir in mushrooms, mayonnaise, Kosher salt and black pepper. Mix in pasta, onion, and tuna.
Transfer mixture to prepared baking dish.
Bake in oven for 30-35 minutes.
Sprinkle cheddar cheese evenly over casserole, then return to oven for an additional 10 minutes.